Cortical Microstimulation and Optogenetics

BOLD sensitivity to cortical activation induced by microstimulation: comparison to visual stimulation

F. Sultan and M. Augath and N. Logothetis

Magn Reson Imaging  25  754-9  (2007)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=17482409

Electrical microstimulation via intracortical electrodes is a widely used method for deducing functions of the brain. In this study, we compared the spatial extent and amplitude of BOLD responses evoked by intracortical electrical stimulation in primary visual cortex with BOLD activations evoked by visual stimulation. The experiments were performed in anesthetized rhesus monkeys. Visual stimulation yielded activities larger than predicted from the well-established visual magnification factor. However, electrical microstimulation yielded an even greater spread of the BOLD response. Our results confirm that the effects of electrical microstimulation extend beyond the brain region expected to be excited by direct current spread.


What delay fields tell us about striate cortex

E. J. Tehovnik and W. M. Slocum

J Neurophysiol  98  559-76  (2007)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=17567774

It is well known that electrical activation of striate cortex (area V1) can disrupt visual behavior. Based on this knowledge, we discovered that electrical microstimulation of V1 in macaque monkeys delays saccadic eye movements when made to visual targets located in the receptive field of the stimulated neurons. This review discusses the following issues. First, the parameters that affect the delay of saccades by microstimulation of V1 are reviewed. Second, the excitability properties of the V1 elements mediating the delay are discussed. Third, the properties that determine the size and shape of the region of visual space affected by stimulation of V1 are described. This region is called a delay field. Fourth, whether the delay effect is mainly due to a disruption of the visual signal transmitted through V1 or whether it is a disturbance of the motor signal transmitted between V1 and the brain stem saccade generator is investigated. Fifth, the properties of delay fields are used to estimate the number of elements activated directly by electrical microstimulation of macaque V1. Sixth, these properties are used to make inferences about the characteristics of visual percepts induced by such stimulation. Seventh, the disruptive effects of V1 stimulation in monkeys and humans are compared. Eighth, a cortical mechanism to account for the disruptive effects of V1 stimulation is proposed. Finally, these effects are related to normal vision.



Doing without learning: stimulation of the frontal eye fields and floccular complex does not instruct motor learning in smooth pursuit eye movements

H. W. Heuer and S. Tokiyama and S. G. Lisberger

J Neurophysiol  100  1320-31  (2008)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=18579657

Under natural conditions, motor learning is instructed by sensory feedback. We have asked whether sensory signals that indicate motor errors are necessary to instruct learning or if the motor signals related to movements normally driven by sensory error signals would be sufficient. We measured eye movements in trained rhesus monkeys while employing electrical microstimulation of the floccular complex of the cerebellum and the smooth eye movement region of the frontal eye fields to alter ongoing pursuit eye movements. Repeated electrical stimulation at fixed times after the onset of target motion and pursuit failed to cause any learning that was retained beyond the time period used to instruct learning. Learning was not uncovered when the target was stabilized with respect to the moving eye to prevent competition between instructive signals created by electrical stimulation and visual image motion signals evoked when stimulation drove the eye away from the tracking target. We suggest that signals emanating from motor-related structures in the pursuit circuit do not instruct learning. Instead, instructive sensory error signals seem to be necessary.



Behavioral time course of microstimulation in cortical area MT

N. Y. Masse and E. P. Cook

J Neurophysiol  103  334-45  (2010)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=19864437

Electrical stimulation of the brain is a valuable research tool and has shown therapeutic promise in the development of new sensory neural prosthetics. Despite its widespread use, we still do not fully understand how current passed through a microelectrode interacts with functioning neural circuits. Past behavioral studies have suggested that weak electrical stimulation (referred to as microstimulation) of sensory areas of cortex produces percepts that are similar to those generated by normal sensory stimuli. In contrast, electrophysiological studies using in vitro or anesthetized preparations have shown that neural activity produced by brief microstimulation is radically different and longer lasting than normal responses. To help reconcile these two aspects of microstimulation, we examined the temporal properties that microstimulation has on visual perception. We found that brief application of subthreshold microstimulation in the middle temporal (MT) area of visual cortex produced smaller and longer-lasting effects on motion perception compared with an equivalent visual stimulus. In agreement with past electrophysiological studies, a computer simulation reproduced our behavioral effects when the time course of a single microstimulation pulse was modeled with three components: an immediate fast strong excitatory component, followed by a weaker inhibitory component, and then followed by a long duration weak excitatory component. Overall, these results suggest the behavioral effects of microstimulation in our experiments were caused by the unique and long-lasting temporal effects microstimulation has on functioning cortical circuits.



The effects of electrical microstimulation on cortical signal propagation

N. K. Logothetis and M. Augath and Y. Murayama and A. Rauch and F. Sultan and J. Goense and A. Oeltermann and H. Merkle

Nat Neurosci  13  1283-91  (2010)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=20818384

Electrical stimulation has been used in animals and humans to study potential causal links between neural activity and specific cognitive functions. Recently, it has found increasing use in electrotherapy and neural prostheses. However, the manner in which electrical stimulation-elicited signals propagate in brain tissues remains unclear. We used combined electrostimulation, neurophysiology, microinjection and functional magnetic resonance imaging (fMRI) to study the cortical activity patterns elicited during stimulation of cortical afferents in monkeys. We found that stimulation of a site in the lateral geniculate nucleus (LGN) increased the fMRI signal in the regions of primary visual cortex (V1) that received input from that site, but suppressed it in the retinotopically matched regions of extrastriate cortex. Consistent with previous observations, intracranial recordings indicated that a short excitatory response occurring immediately after a stimulation pulse was followed by a long-lasting inhibition. Following microinjections of GABA antagonists in V1, LGN stimulation induced positive fMRI signals in all of the cortical areas. Taken together, our findings suggest that electrical stimulation disrupts cortico-cortical signal propagation by silencing the output of any neocortical area whose afferents are electrically stimulated.



Direct comparison of spontaneous functional connectivity and effective connectivity measured by intracortical microstimulation: an fMRI study in macaque monkeys

T. Matsui and K. Tamura and K. W. Koyano and D. Takeuchi and Y. Adachi and T. Osada and Y. Miyashita

Cereb Cortex  21  2348-56  (2011)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=21368090

Correlated spontaneous activity in the resting brain is increasingly recognized as a useful index for inferring underlying functional-anatomic architecture. However, despite efforts for comparison with anatomical connectivity, neuronal origin of intrinsic functional connectivity (inFC) remains unclear. Conceptually, the source of inFC could be decomposed into causal components that reflect the efficacy of synaptic interactions and other components mediated by collective network dynamics (e.g., synchronization). To dissociate these components, it is useful to introduce another connectivity measure such as effective connectivity, which is a quantitative measure of causal interactions. Here, we present a direct comparison of inFC against emEC (effective connectivity probed with electrical microstimulation [EM]) in the somatosensory system of macaque monkeys. Simultaneous EM and functional magnetic resonance imaging revealed strong emEC in several brain regions in a manner consistent with the anatomy of somatosensory system. Direct comparison of inFC and emEC revealed colocalization and overall positive correlation within the stimulated hemisphere. Interestingly, we found characteristic differences between inFC and emEC in their interhemispheric patterns. Our results suggest that intrahemispheric inFC reflects the efficacy of causal interactions, whereas interhemispheric inFC may arise from interactions akin to network-level synchronization that is not captured by emEC.



Current-distance relations for microelectrode stimulation of pyramidal cells

C. Wenger and L. Paredes and F. Rattay

Artif Organs  35  263-6  (2011)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=21401671

Microelectrodes placed within the densely packed cortical neuronal region are surrounded by many thin processes. Although dendrites are considered to be functionally different to axons, they also possess voltage sensitive membrane channels. Therefore, dendritic regions are suitable candidates for spike initiation sites when stimulated externally, although they demand two to three times higher thresholds in comparison with thin axons. Simulations based upon recently reported distributions of two types of sodium channels and traced pyramidal cell data accompanied by a simplified model structure enlightened the spike initiation sites for extracellular cortical microstimulation and revealed insights into dendritic excitation patterns. Surprisingly low dendritic threshold values for cathodic stimulation were detected, that is, 3.3 µA for a 0.4-µm diameter fiber excited with a 100-µs pulse in 4-µm distance. However, according to the activating function concept the excited region is calculated by 1414*electrode-distance, therefore a minimum electrode-fiber distance is required as sufficient sodium channels are needed to produce enough intracellular current for spike conduction. The minimum distance for dendritic spike initiation increases with diameter and hinders low current stimulation of thick dendrites. This effect is in contrast to the inverse recruitment order known from functional electrical stimulation. Simulations were performed using NEURON and MATLAB.



Optical imaging in galagos reveals parietal-frontal circuits underlying motor behavior

I. Stepniewska and R. M. Friedman and O. A. Gharbawie and C. M. Cerkevich and A. W. Roe and J. H. Kaas

Proc Natl Acad Sci U S A  108  E725-32  (2011)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=21873212

The posterior parietal cortex (PPC) of monkeys and prosimian galagos contains a number of subregions where complex, behaviorally meaningful movements, such as reaching, grasping, and body defense, can be evoked by electrical stimulation with long trains of electrical pulses through microelectrodes. Shorter trains of pulses evoke no or simple movements. One possibility for the difference in effectiveness of intracortical microstimulation is that long trains activate much larger regions of the brain. Here, we show that long-train stimulation of PPC does not activate widespread regions of frontal motor and premotor cortex but instead, produces focal, somatotopically appropriate activations of frontal motor and premotor cortex. Shorter stimulation trains activate the same frontal foci but less strongly, showing that longer stimulus trains do not produce less specification. Because the activated sites in frontal cortex correspond to the locations of direct parietal-frontal anatomical connections from the stimulated PPC subregions, the results show the usefulness of optical imaging in conjunction with electrical stimulation in showing functional pathways between nodes in behavior-specific cortical networks. Thus, long-train stimulation is effective in evoking ethologically relevant sequences of movements by activating nodes in a cortical network for a behaviorally relevant period rather than spreading activation in a nonspecific manner.



Localized microstimulation of primate pregenual cingulate cortex induces negative decision-making

K.-i. Amemori and A. M. Graybiel

Nat Neurosci  15  776-85  (2012)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=22484571

The pregenual anterior cingulate cortex (pACC) has been implicated in human anxiety disorders and depression, but the circuit-level mechanisms underlying these disorders are unclear. In healthy individuals, the pACC is involved in cost-benefit evaluation. We developed a macaque version of an approach-avoidance decision task used to evaluate anxiety and depression in humans and, with multi-electrode recording and cortical microstimulation, we probed pACC function as monkeys performed this task. We found that the macaque pACC has an opponent process-like organization of neurons representing motivationally positive and negative subjective value. Spatial distribution of these two neuronal populations overlapped in the pACC, except in one subzone, where neurons with negative coding were more numerous. Notably, microstimulation in this subzone, but not elsewhere in the pACC, increased negative decision-making, and this negative biasing was blocked by anti-anxiety drug treatment. This cortical zone could be critical for regulating negative emotional valence and anxiety in decision-making.



Optogenetic manipulation of cerebellar Purkinje cell activity in vivo

T. Tsubota and Y. Ohashi and K. Tamura and A. Sato and Y. Miyashita

PLoS One  6  e22400  (2011)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=21850224

Purkinje cells (PCs) are the sole output neurons of the cerebellar cortex. Although their anatomical connections and physiological response properties have been extensively studied, the causal role of their activity in behavioral, cognitive and autonomic functions is still unclear because PC activity cannot be selectively controlled. Here we developed a novel technique using optogenetics for selective and rapidly reversible manipulation of PC activity in vivo. We injected into rat cerebellar cortex lentiviruses expressing either the light-activated cationic channel channelrhodopsin-2 (ChR2) or light-driven chloride pump halorhodopsin (eNpHR) under the control of the PC-specific L7 promoter. Transgene expression was observed in most PCs (ChR2, 92.6%; eNpHR, 95.3%), as determined by immunohistochemical analysis. In vivo electrophysiological recordings showed that all light-responsive PCs in ChR2-transduced rats increased frequency of simple spike in response to blue laser illumination. Similarly, most light-responsive PCs (93.8%) in eNpHR-transduced rats decreased frequency of simple spike in response to orange laser illumination. We then applied these techniques to characterize the roles of rat cerebellar uvula, one of the cardiovascular regulatory regions in the cerebellum, in resting blood pressure (BP) regulation in anesthetized rats. ChR2-mediated photostimulation and eNpHR-mediated photoinhibition of the uvula had opposite effects on resting BP, inducing depressor and pressor responses, respectively. In contrast, manipulation of PC activity within the neighboring lobule VIII had no effect on BP. Blue and orange laser illumination onto PBS-injected lobule IX didn't affect BP, indicating the observed effects on BP were actually due to PC activation and inhibition. These results clearly demonstrate that the optogenetic method we developed here will provide a powerful way to elucidate a causal relationship between local PC activity and functions of the cerebellum.



A high-light sensitivity optical neural silencer: development and application to optogenetic control of non-human primate cortex

X. Han and B. Y. Chow and H. Zhou and N. C. Klapoetke and A. Chuong and R. Rajimehr and A. Yang and M. V. Baratta and J. Winkle and R. Desimone and E. S. Boyden

Front Syst Neurosci  5  18  (2011)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=21811444

Technologies for silencing the electrical activity of genetically targeted neurons in the brain are important for assessing the contribution of specific cell types and pathways toward behaviors and pathologies. Recently we found that archaerhodopsin-3 from Halorubrum sodomense (Arch), a light-driven outward proton pump, when genetically expressed in neurons, enables them to be powerfully, transiently, and repeatedly silenced in response to pulses of light. Because of the impressive characteristics of Arch, we explored the optogenetic utility of opsins with high sequence homology to Arch, from archaea of the Halorubrum genus. We found that the archaerhodopsin from Halorubrum strain TP009, which we named ArchT, could mediate photocurrents of similar maximum amplitude to those of Arch (900pA in vitro), but with a >3-fold improvement in light sensitivity over Arch, most notably in the optogenetic range of 1-10 mW/mm(2), equating to >2× increase in brain tissue volume addressed by a typical single optical fiber. Upon expression in mouse or rhesus macaque cortical neurons, ArchT expressed well on neuronal membranes, including excellent trafficking for long distances down neuronal axons. The high light sensitivity prompted us to explore ArchT use in the cortex of the rhesus macaque. Optical perturbation of ArchT-expressing neurons in the brain of an awake rhesus macaque resulted in a rapid and complete (100%) silencing of most recorded cells, with suppressed cells achieving a median firing rate of 0 spikes/s upon illumination. A small population of neurons showed increased firing rates at long latencies following the onset of light stimulation, suggesting the existence of a mechanism of network-level neural activity balancing. The powerful net suppression of activity suggests that ArchT silencing technology might be of great use not only in the causal analysis of neural circuits, but may have therapeutic applications.



Motor cortex stimulation for Parkinson's disease: a modelling study

D. G. M. Zwartjes and T. Heida and H. K. P. Feirabend and M. L. F. Janssen and V. Visser-Vandewalle and H. C. F. Martens and P. H. Veltink

J Neural Eng  9  056005  (2012)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=22878550

Chronic motor cortex stimulation (MCS) is currently being investigated as a treatment method for Parkinson's disease (PD). Unfortunately, the underlying mechanisms of this treatment are unclear and there are many uncertainties regarding the most effective stimulation parameters and electrode configuration. In this paper, we present a MCS model with a 3D representation of several axonal populations. The model predicts that the activation of either the basket cell or pyramidal tract (PT) type axons is involved in the clinical effect of MCS. We propose stimulation protocols selectively targeting one of these two axon types. To selectively target the basket cell axons, our simulations suggest using either cathodal or bipolar stimulation with the electrode strip placed perpendicular rather than parallel to the gyrus. Furthermore, selectivity can be increased by using multiple cathodes. PT type axons can be selectively targeted with anodal stimulation using electrodes with large contact sizes. Placing the electrode epidurally is advisable over subdural placement. These selective protocols, when practically implemented, can be used to further test which axon type should be activated for clinically effective MCS and can subsequently be applied to optimize treatment. In conclusion, this paper increases insight into the neuronal population involved in the clinical effect of MCS on PD and proposes strategies to improve this therapy.



Stimulation with a low-amplitude, digitized synaptic signal to invoke robust activity within neuronal networks on multielectrode arrays

J. M. Zemianek and M. Serra and M. Guaraldi and T. B. Shea

Biotechniques  52  177-82  (2012)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=22401551

Multielectrode arrays (MEAs) are used for analysis of neuronal activity. Here we report two variations on commonly accepted techniques that increase the precision of extracellular electrical stimulation: (i) the use of a low-amplitude recorded spontaneous synaptic signal as a stimulus waveform and (ii) the use of a specific electrode within the array adjacent to the stimulus electrode as a hard-grounded stimulus signal return path. Both modifications remained compatible with manipulation of neuronal networks. In addition, localized stimulation with the low-amplitude synaptic signal allowed selective stimulation or inhibition of otherwise spontaneous signals. These findings indicate that minimizing the area of the culture impacted by external stimulation allows modulation of signaling patterns within subpopulations of neurons in culture. The simple modifications described herein may be useful for precise monitoring and manipulation of neuronal networks.



Microstimulation of V1 input layers disrupts the selection and detection of visual targets by monkeys

W. M. Slocum and E. J. Tehovnik

Eur J Neurosci  20  1674-80  (2004)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=15355335

Electrical microstimulation delivered to primary visual cortex (V1) concurrently with the presentation of visual targets interferes with the selection of these targets. To determine the source of this interference, we stimulated the visual input layers of V1 as rhesus monkeys generated saccadic eye movements to visual targets presented at and outside the receptive field of the stimulated neurons. Columns of cells in V1 innervated by the left and right eye are segregated according to eye dominance, such that cells within a column respond best to visual stimuli presented to the ocular dominant eye. Interference was maximal when targets were presented to the ocular dominant eye, moderate when presented to the ocular inferior eye, and negligible when presented to both eyes. Thus, electrical microstimulation of the visual input layers of V1 disrupts the flow of visual information along the geniculostriate pathway. Knowing how electrical stimulation of V1 affects visual behaviour is necessary when using monkeys to develop a visual prosthesis for the blind.



Learning to recognize visual objects with microstimulation in inferior temporal cortex

K. Kawasaki and D. L. Sheinberg

J Neurophysiol  100  197-211  (2008)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=18463185

The malleability of object representations by experience is essential for adaptive behavior. It has been hypothesized that neurons in inferior temporal cortex (IT) in monkeys are pivotal in visual association learning, evidenced by experiments revealing changes in neural selectivity following visual learning, as well as by lesion studies, wherein functional inactivation of IT impairs learning. A critical question remaining to be answered is whether IT neuronal activity is sufficient for learning. To address this question directly, we conducted experiments combining visual classification learning with microstimulation in IT. We assessed the effects of IT microstimulation during learning in cases where the stimulation was exclusively informative, conditionally informative, and informative but not necessary for the classification task. The results show that localized microstimulation in IT can be used to establish visual classification learning, and the same stimulation applied during learning can predictably bias judgments on subsequent recognition. The effect of induced activity can be explained neither by direct stimulation-motor association nor by simple detection of cortical stimulation. We also found that the learning effects are specific to IT stimulation as they are not observed by microstimulation in an adjacent auditory area. Our results add the evidence that the differential activity in IT during visual association learning is sufficient for establishing new associations. The results suggest that experimentally manipulated activity patterns within IT can be effectively combined with ongoing visually induced activity during the formation of new associations.



Enhanced detection threshold for in vivo cortical stimulation produced by Hebbian conditioning

J. M. Rebesco and L. E. Miller

J Neural Eng  8  016011  (2011)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=21252415

Normal brain function requires constant adaptation, as an organism learns to associate important sensory stimuli with the appropriate motor actions. Neurological disorders may disrupt these learned associations and require the nervous system to reorganize itself. As a consequence, neural plasticity is a crucial component of normal brain function and a critical mechanism for recovery from injury. Associative, or Hebbian, pairing of pre- and post-synaptic activity has been shown to alter stimulus-evoked responses in vivo; however, to date, such protocols have not been shown to affect the animal's subsequent behavior. We paired stimulus trains separated by a brief time delay to two electrodes in rat sensorimotor cortex, which changed the statistical pattern of spikes during subsequent behavior. These changes were consistent with strengthened functional connections from the leading electrode to the lagging electrode. We then trained rats to respond to a microstimulation cue, and repeated the paradigm using the cue electrode as the leading electrode. This pairing lowered the rat's ICMS-detection threshold, with the same dependence on intra-electrode time lag that we found for the functional connectivity changes. The timecourse of the behavioral effects was very similar to that of the connectivity changes. We propose that the behavioral changes were a consequence of strengthened functional connections from the cue electrode to other regions of sensorimotor cortex. Such paradigms might be used to augment recovery from a stroke, or to promote adaptation in a bidirectional brain-machine interface.



Driving opposing behaviors with ensembles of piriform neurons

G. B. Choi and D. D. Stettler and B. R. Kallman and S. T. Bhaskar and A. Fleischmann and R. Axel

Cell  146  1004-15  (2011)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=21925321

Anatomic and physiologic studies have suggested a model in which neurons of the piriform cortex receive convergent input from random collections of glomeruli. In this model, odor representations can only be afforded behavioral significance upon experience. We have devised an experimental strategy that permits us to ask whether the activation of an arbitrarily chosen subpopulation of neurons in piriform cortex can elicit different behavioral responses dependent upon learning. Activation of a small subpopulation of piriform neurons expressing channelrhodopsin at multiple loci in the piriform cortex, when paired with reward or shock, elicits either appetitive or aversive behavior. Moreover, we demonstrate that different subpopulations of piriform neurons expressing ChR2 can be discriminated and independently entrained to elicit distinct behaviors. These observations demonstrate that the piriform cortex is sufficient to elicit learned behavioral outputs in the absence of sensory input. These data imply that the piriform does not use spatial order to map odorant identity or behavioral output.



Behavioral detection of electrical microstimulation in different cortical visual areas.

D. K. Murphey and J. H. R. Maunsell

Curr Biol  17  862--867  (2007)



http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=17462895

The extent to which areas in the visual cerebral cortex differ in their ability to support perceptions has been the subject of considerable speculation. Experiments examining the activity of individual neurons have suggested that activity in later stages of the visual cortex is more closely linked to perception than that in earlier stages [1-9]. In contrast, results from functional imaging, transcranial magnetic stimulation, and lesion studies have been interpreted as showing that earlier stages are more closely coupled to perception [10-15]. We examined whether neuronal activity in early and later stages differs in its ability to support detectable signals by measuring behavioral thresholds for detecting electrical microstimulation in different cortical areas in two monkeys. By training the animals to perform a two-alternative temporal forced-choice task, we obtained criterion-free thresholds from five visual areas--V1, V2, V3A, MT, and the inferotemporal cortex. Every site tested yielded a reliable threshold. Thresholds varied little within and between visual areas, rising gradually from early to later stages. We similarly found no systematic differences in the slopes of the psychometric detection functions from different areas. These results suggest that neuronal signals of similar magnitude evoked in any part of visual cortex can generate percepts.



Microstimulation reveals limits in detecting different signals from a local cortical region

A. M. Ni and J. H. R. Maunsell

Curr Biol  20  824-8  (2010)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=20381351

Behavioral performance depends on the activity of neurons in sensory cortex, but little is known about the brain's capacity to access specific neuronal signals to guide behavior. Even the individual sensory neurons that are most sensitive to a relevant stimulus are only weakly correlated with behavior, suggesting that behavioral decisions are based on the combined activity of groups of neurons with sensitivities well matched to task demands. To explore how flexibly different patterns of activity can be accessed from a given cortical region, we trained animals to detect electrical microstimulation of local V1 sites. By allowing the animals to become expert at the detection of microstimulation of specific V1 sites that corresponded to particular retinotopic locations, we could measure the effects of that training on the ability of those sites to support the detection of visual stimuli. Training to detect electrical activation caused a large, reversible, retinotopically localized impairment of thresholds for detecting visual stimuli. Retraining on visual detection restored normal thresholds and in turn impaired thresholds for detecting microstimulation. These results suggest that there are substantial limits to the types of signals for which a local cortical region can be simultaneously optimized.



Optogenetic tools for analyzing the neural circuits of behavior

J. G. Bernstein and E. S. Boyden

Trends Cogn Sci  15  592-600  (2011)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=22055387

In order to understand how the brain generates behaviors, it is important to be able to determine how neural circuits work together to perform computations. Because neural circuits are made of a great diversity of cell types, it is critical to be able to analyze how these different kinds of cell work together. In recent years, a toolbox of fully genetically encoded molecules has emerged that, when expressed in specific neurons, enables the electrical activity of the targeted neurons to be controlled in a temporally precise fashion by pulses of light. We describe this optogenetic toolbox, how it can be used to analyze neural circuits in the brain and how optogenetics is impacting the study of cognition.



Optogenetic manipulation of neural circuitry in vivo

A. V. Kravitz and A. C. Kreitzer

Curr Opin Neurobiol  21  433-9  (2011)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=21420852

Recent advances in optogenetics have permitted investigations of specific cell types in the nervous system with unprecedented precision and control. This review will discuss the use of optogenetic techniques in the study of mammalian neural circuitry in vivo, as well as practical and theoretical considerations in their application.



Multi-array silicon probes with integrated optical fibers: light-assisted perturbation and recording of local neural circuits in the behaving animal

S. Royer and B. V. Zemelman and M. Barbic and A. Losonczy and G. Buzsáki and J. C. Magee

Eur J Neurosci  31  2279-91  (2010)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=20529127

Recordings of large neuronal ensembles and neural stimulation of high spatial and temporal precision are important requisites for studying the real-time dynamics of neural networks. Multiple-shank silicon probes enable large-scale monitoring of individual neurons. Optical stimulation of genetically targeted neurons expressing light-sensitive channels or other fast (milliseconds) actuators offers the means for controlled perturbation of local circuits. Here we describe a method to equip the shanks of silicon probes with micron-scale light guides for allowing the simultaneous use of the two approaches. We then show illustrative examples of how these compact hybrid electrodes can be used in probing local circuits in behaving rats and mice. A key advantage of these devices is the enhanced spatial precision of stimulation that is achieved by delivering light close to the recording sites of the probe. When paired with the expression of light-sensitive actuators within genetically specified neuronal populations, these devices allow the relatively straightforward and interpretable manipulation of network activity.



Which elements of the mammalian central nervous system are excited by low current stimulation with microelectrodes?

F. Rattay and C. Wenger

Neuroscience  170  399-407  (2010)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=20659531

Low current cortex stimulation produces a sparse and distributed set of activated cells often with distances of several hundred micrometers between cell bodies and the microelectrode. A modeling study based on recently measured densities of high threshold sodium channels Nav1.2 in dendrites and soma and low threshold sodium channels Nav1.6 in the axon shall identify spike initiation sites including a discussion on dendritic spikes. Varying excitability along the neural axis has been observed while studying different electrode positions and configurations. Although the axon initial segment (AIS) and nodes of Ranvier are most excitable, many thin axons and dendrites which are likely to be close to the electrode in the densely packed cortical regions are also proper candidates for spike initiation sites. Cathodic threshold ratio for thin axons and dendrites is about 1:3, whereas 0.2 mum diameter axons passing the electrode tip in 10 mum distance can be activated by 100 mus pulses with 2.6 muA. Direct cathodic excitation of dendrites requires a minimum electrode-fiber distance, which increases with dendrite diameter. Therefore thin dendrites can profit from the stronger electrical field close to the electrode but low current stimulation cannot activate large diameter dendrites, contrary to the inverse recruitment order known from peripheral nerve stimulation. When local depolarization fails to generate a dendritic spike, stimulation is possible via intracellular current flow that initiates an action potential, for example 200 mum distant in the low threshold AIS or in certain cases at the distal dendrite ending. Beside these exceptions, spike initiation site for cathodic low current stimulation appears rather close to the electrode.



Rewiring neural interactions by micro-stimulation

J. M. Rebesco and I. H. Stevenson and K. P. Körding and S. A. Solla and L. E. Miller

Front Syst Neurosci  4    (2010)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=20838477

Plasticity is a crucial component of normal brain function and a critical mechanism for recovery from injury. In vitro, associative pairing of presynaptic spiking and stimulus-induced postsynaptic depolarization causes changes in the synaptic efficacy of the presynaptic neuron, when activated by extrinsic stimulation. In vivo, such paradigms can alter the responses of whole groups of neurons to stimulation. Here, we used in vivo spike-triggered stimulation to drive plastic changes in rat forelimb sensorimotor cortex, which we monitored using a statistical measure of functional connectivity inferred from the spiking statistics of the neurons during normal, spontaneous behavior. These induced plastic changes in inferred functional connectivity depended on the latency between trigger spike and stimulation, and appear to reflect a robust reorganization of the network. Such targeted connectivity changes might provide a tool for rerouting the flow of information through a network, with implications for both rehabilitation and brain-machine interface applications.



Probing neural circuitry and function with electrical microstimulation

K. L. Clark and K. M. Armstrong and T. Moore

Proc Biol Sci  278  1121-30  (2011)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=21247952

Since the discovery of the nervous system's electrical excitability more than 200 years ago, neuroscientists have used electrical stimulation to manipulate brain activity in order to study its function. Microstimulation has been a valuable technique for probing neural circuitry and identifying networks of neurons that underlie perception, movement and cognition. In this review, we focus on the use of stimulation in behaving primates, an experimental system that permits causal inferences to be made about the effect of stimulation-induced activity on the resulting behaviour or neural signals elsewhere in the brain.



Viral vector-based reversible neuronal inactivation and behavioral manipulation in the macaque monkey

K. J. Nielsen and E. M. Callaway and R. J. Krauzlis

Front Syst Neurosci  6  48  (2012)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=22723770

Viral vectors are promising tools for the dissection of neural circuits. In principle, they can manipulate neurons at a level of specificity not otherwise achievable. While many studies have used viral vector-based approaches in the rodent brain, only a few have employed this technique in the non-human primate, despite the importance of this animal model for neuroscience research. Here, we report evidence that a viral vector-based approach can be used to manipulate a monkey's behavior in a task. For this purpose, we used the allatostatin receptor/allatostatin (AlstR/AL) system, which has previously been shown to allow inactivation of neurons in vivo. The AlstR was expressed in neurons in monkey V1 by injection of an adeno-associated virus 1 (AAV1) vector. Two monkeys were trained in a detection task, in which they had to make a saccade to a faint peripheral target. Injection of AL caused a retinotopic deficit in the detection task in one monkey. Specifically, the monkey showed marked impairment for detection targets placed at the visual field location represented at the virus injection site, but not for targets shown elsewhere. We confirmed that these deficits indeed were due to the interaction of AlstR and AL by injecting saline, or AL at a V1 location without AlstR expression. Post-mortem histology confirmed AlstR expression in this monkey. We failed to replicate the behavioral results in a second monkey, as AL injection did not impair the second monkey's performance in the detection task. However, post-mortem histology revealed a very low level of AlstR expression in this monkey. Our results demonstrate that viral vector-based approaches can produce effects strong enough to influence a monkey's performance in a behavioral task, supporting the further development of this approach for studying how neuronal circuits control complex behaviors in non-human primates.



An optogenetic toolbox designed for primates

I. Diester and M. T. Kaufman and M. Mogri and R. Pashaie and W. Goo and O. Yizhar and C. Ramakrishnan and K. Deisseroth and K. V. Shenoy

Nat Neurosci  14  387-97  (2011)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=21278729

Optogenetics is a technique for controlling subpopulations of neurons in the intact brain using light. This technique has the potential to enhance basic systems neuroscience research and to inform the mechanisms and treatment of brain injury and disease. Before launching large-scale primate studies, the method needs to be further characterized and adapted for use in the primate brain. We assessed the safety and efficiency of two viral vector systems (lentivirus and adeno-associated virus), two human promoters (human synapsin (hSyn) and human thymocyte-1 (hThy-1)) and three excitatory and inhibitory mammalian codon-optimized opsins (channelrhodopsin-2, enhanced Natronomonas pharaonis halorhodopsin and the step-function opsin), which we characterized electrophysiologically, histologically and behaviorally in rhesus monkeys (Macaca mulatta). We also introduced a new device for measuring in vivo fluorescence over time, allowing minimally invasive assessment of construct expression in the intact brain. We present a set of optogenetic tools designed for optogenetic experiments in the non-human primate brain.



Designing optimal stimuli to control neuronal spike timing

Y. Ahmadian and A. M. Packer and R. Yuste and L. Paninski

J Neurophysiol  106  1038-53  (2011)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=21511704

Recent advances in experimental stimulation methods have raised the following important computational question: how can we choose a stimulus that will drive a neuron to output a target spike train with optimal precision, given physiological constraints? Here we adopt an approach based on models that describe how a stimulating agent (such as an injected electrical current or a laser light interacting with caged neurotransmitters or photosensitive ion channels) affects the spiking activity of neurons. Based on these models, we solve the reverse problem of finding the best time-dependent modulation of the input, subject to hardware limitations as well as physiologically inspired safety measures, that causes the neuron to emit a spike train that with highest probability will be close to a target spike train. We adopt fast convex constrained optimization methods to solve this problem. Our methods can potentially be implemented in real time and may also be generalized to the case of many cells, suitable for neural prosthesis applications. With the use of biologically sensible parameters and constraints, our method finds stimulation patterns that generate very precise spike trains in simulated experiments. We also tested the intracellular current injection method on pyramidal cells in mouse cortical slices, quantifying the dependence of spiking reliability and timing precision on constraints imposed on the applied currents.



Ventral premotor-motor cortex interactions in the macaque monkey during grasp: response of single neurons to intracortical microstimulation

A. Kraskov and G. Prabhu and M. M. Quallo and R. N. Lemon and T. Brochier

J Neurosci  31  8812-21  (2011)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=21677165

Recent stimulation studies in monkeys and humans have shown strong interactions between ventral premotor cortex (area F5) and the hand area of primary motor cortex (M1). These short-latency interactions usually involve facilitation from F5 of M1 outputs to hand muscles, although suppression has also been reported. This study, performed in three awake macaque monkeys, sought evidence that these interactions could be mediated by short-latency excitatory and inhibitory responses of single M1 neurons active during grasping tasks. We recorded responses of these M1 neurons to single low-threshold (≤40 μA) intracortical microstimuli delivered to F5 sites at which grasp-related neurons were recorded. In 29 sessions, we tested 232 M1 neurons with stimuli delivered to between one and four sites in F5. Of the 415 responses recorded, 142 (34%) showed significant effects. The most common type of response was pure excitation (53% of responses), with short latency (1.8-3.0 ms) and brief duration (1 ms); purely inhibitory responses had slightly longer latencies (2-5 ms) and were of small amplitude and longer duration (5-7 ms). They accounted for 13% of responses, whereas mixed excitation then inhibition was seen in 34%. Remarkably, a rather similar set of findings applied to 280 responses of 138 F5 neurons to M1 stimulation; 109 (34%) responses showed significant effects. Thus, with low-intensity stimuli, the dominant interaction between these two cortical areas is one of short-latency, brief excitation, most likely mediated by reciprocal F5-M1 connections. Some neurons were tested with stimuli at both 20 and 40 μA; inhibition tended to dominate at the higher intensity.


Hijacking cortical motor output with repetitive microstimulation

D. M. Griffin and H. M. Hudson and A. Belhaj-Saïf and P. D. Cheney

J Neurosci  31  13088-96  (2011)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=21917792

High-frequency repetitive microstimulation has been widely used as a method of investigating the properties of cortical motor output. Despite its widespread use, few studies have investigated how activity evoked by high-frequency stimulation may interact with the existing activity of cortical cells resulting from natural synaptic inputs. A reasonable assumption might be that the stimulus-evoked activity sums with the existing natural activity. However, another possibility is that the stimulus-evoked firing of cortical neurons might block and replace the natural activity. We refer to this latter possibility as "neural hijacking." Evidence from analysis of EMG activity evoked by repetitive microstimulation (200 Hz, 500 ms) of primary motor cortex in two rhesus monkeys during performance of a reach-to-grasp task strongly supports the neural hijacking hypothesis.


Extracellular neural microstimulation may activate much larger regions than expected by simulations: a combined experimental and modeling study

S. Joucla and P. Branchereau and D. Cattaert and B. Yvert

PLoS One  7  e41324  (2012)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=22879886

Electrical stimulation of the central nervous system has been widely used for decades for either fundamental research purposes or clinical treatment applications. Yet, very little is known regarding the spatial extent of an electrical stimulation. If pioneering experimental studies reported that activation threshold currents (TCs) increase with the square of the neuron-to-electrode distance over a few hundreds of microns, there is no evidence that this quadratic law remains valid for larger distances. Moreover, nowadays, numerical simulation approaches have supplanted experimental studies for estimating TCs. However, model predictions have not yet been validated directly with experiments within a common paradigm. Here, we present a direct comparison between experimental determination and modeling prediction of TCs up to distances of several millimeters. First, we combined patch-clamp recording and microelectrode array stimulation in whole embryonic mouse spinal cords to determine TCs. Experimental thresholds did not follow a quadratic law beyond 1 millimeter, but rather tended to remain constant for distances larger than 1 millimeter. We next built a combined finite element - compartment model of the same experimental paradigm to predict TCs. While theoretical TCs closely matched experimental TCs for distances <250 microns, they were highly overestimated for larger distances. This discrepancy remained even after modifications of the finite element model of the potential field, taking into account anisotropic, heterogeneous or dielectric properties of the tissue. In conclusion, these results show that quadratic evolution of TCs does not always hold for large distances between the electrode and the neuron and that classical models may underestimate volumes of tissue activated by electrical stimulation.


Mapping cortical activity elicited with electrical microstimulation using FMRI in the macaque.

A. S. Tolias and F. Sultan and M. Augath and A. Oeltermann and E. J. Tehovnik and P. H. Schiller and N. K. Logothetis

Neuron  48  901--911  (2005)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=16364895

Over the last two centuries, electrical microstimulation has been used to demonstrate causal links between neural activity and specific behaviors and cognitive functions. However, to establish these links it is imperative to characterize the cortical activity patterns that are elicited by stimulation locally around the electrode and in other functionally connected areas. We have developed a technique to record brain activity using the blood oxygen level dependent (BOLD) signal while applying electrical microstimulation to the primate brain. We find that the spread of activity around the electrode tip in macaque area V1 was larger than expected from calculations based on passive spread of current and therefore may reflect functional spread by way of horizontal connections. Consistent with this functional transynaptic spread we also obtained activation in expected projection sites in extrastriate visual areas, demonstrating the utility of our technique in uncovering in vivo functional connectivity maps.



Behavioural report of single neuron stimulation in somatosensory cortex

A. R. Houweling and M. Brecht

Nature  451  65-8  (2008)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=18094684

Understanding how neural activity in sensory cortices relates to perception is a central theme of neuroscience. Action potentials of sensory cortical neurons can be strongly correlated to properties of sensory stimuli and reflect the subjective judgements of an individual about stimuli. Microstimulation experiments have established a direct link from sensory activity to behaviour, suggesting that small neuronal populations can influence sensory decisions. However, microstimulation does not allow identification and quantification of the stimulated cellular elements. The sensory impact of individual cortical neurons therefore remains unknown. Here we show that stimulation of single neurons in somatosensory cortex affects behavioural responses in a detection task. We trained rats to respond to microstimulation of barrel cortex at low current intensities. We then initiated short trains of action potentials in single neurons by juxtacellular stimulation. Animals responded significantly more often in single-cell stimulation trials than in catch trials without stimulation. Stimulation effects varied greatly between cells, and on average in 5% of trials a response was induced. Whereas stimulation of putative excitatory neurons led to weak biases towards responding, stimulation of putative inhibitory neurons led to more variable and stronger sensory effects. Reaction times for single-cell stimulation were long and variable. Our results demonstrate that single neuron activity can cause a change in the animal's detection behaviour, suggesting a much sparser cortical code for sensations than previously anticipated.



Sparse and powerful cortical spikes

J. Wolfe and A. R. Houweling and M. Brecht

Curr Opin Neurobiol  20  306-12  (2010)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=20400290

Activity in cortical networks is heterogeneous, sparse and often precisely timed. The functional significance of sparseness and precise spike timing is debated, but our understanding of the developmental and synaptic mechanisms that shape neuronal discharge patterns has improved. Evidence for highly specialized, selective and abstract cortical response properties is accumulating. Singe-cell stimulation experiments demonstrate a high sensitivity of cortical networks to the action potentials of some, but not all, single neurons. It is unclear how this sensitivity of cortical networks to small perturbations comes about and whether it is a generic property of cortex. The unforeseen sensitivity to cortical spikes puts serious constraints on the nature of neural coding schemes.



Nanostimulation: manipulation of single neuron activity by juxtacellular current injection

A. R. Houweling and G. Doron and B. C. Voigt and L. J. Herfst and M. Brecht

J Neurophysiol  103  1696-704  (2010)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=19955285

In the mammalian brain, many thousands of single-neuron recording studies have been performed but less than 10 single-cell stimulation studies. This paucity of single-cell stimulation data reflects a lack of easily applicable single-cell stimulation techniques. We provide a detailed description of the procedures involved in nanostimulation, a single-cell stimulation method derived from the juxtacellular labeling technique. Nanostimulation is easy to apply and can be directed to a wide variety of identifiable neurons in anesthetized and awake animals. We describe the recording approach and the parameters of the electric configuration underlying nanostimulation. We use glass pipettes with a DC resistance of 4-7 Mohms. Obtaining the juxtacellular configuration requires a close contact between pipette tip and neuron and is associated with a several-fold increase in resistance to values > or = 20 Mohms. The recorded action potential (AP) amplitude grows to > or = 2 mV, and neurons can be activated with currents in the nanoampere range--hence the term nanostimulation. While exact AP timing has not been achieved, AP frequency and AP number can be parametrically controlled. We demonstrate that nanostimulation can also be used to selectively inhibit sensory responses in identifiable neurons. Nanostimulation is biophysically similar to electroporation, and based on this assumption, we argue that nanostimulation operates on membranes in the micrometer area directly below the pipette tip, where membrane pores are induced by high transmembrane voltage. There is strong evidence to suggest that nanostimulation selectively activates single neurons and that the evoked effects are cell-specific. Nanostimulation therefore holds great potential for elucidating how single neurons contribute to behavior.



What electrical microstimulation has revealed about the neural basis of cognition.

M. R. Cohen and W. T. Newsome

Curr Opin Neurobiol  14  169--177  (2004)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=15082321

Neurophysiologists have shown repeatedly that neural activity in different brain structures can be correlated with specific perceptual and cognitive functions, but the causal efficacy of the observed activity has generally been a matter of conjecture. By contrast, electrical microstimulation, which allows the experimenter to manipulate the activity of small groups of neurons with spatial and temporal precision, can now be used to demonstrate causal links between neural activity and specific cognitive functions. Here, we review this growing literature, including applications to the study of attention, visual and somatosensory perception, 'read-out' mechanisms for interpreting sensory maps, and contextual effects on perception. We also discuss potential applications of microstimulation to studies of higher cognitive functions such as decision-making and subjective experience.



Direct and indirect activation of cortical neurons by electrical microstimulation.

E. J. Tehovnik and A. S. Tolias and F. Sultan and W. M. Slocum and N. K. Logothetis

J Neurophysiol  96  512--521  (2006)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=16835359

Electrical microstimulation has been used to elucidate cortical function. This review discusses neuronal excitability and effective current spread estimated by using three different methods: 1) single-cell recording, 2) behavioral methods, and 3) functional magnetic resonance imaging (fMRI). The excitability properties of the stimulated elements in neocortex obtained using these methods were found to be comparable. These properties suggested that microstimulation activates the most excitable elements in cortex, that is, by and large the fibers of the pyramidal cells. Effective current spread within neocortex was found to be greater when measured with fMRI compared with measures based on single-cell recording or behavioral methods. The spread of activity based on behavioral methods is in close agreement with the spread based on the direct activation of neurons (as opposed to those activated synaptically). We argue that the greater activation with imaging is attributed to transynaptic spread, which includes subthreshold activation of sites connected to the site of stimulation. The definition of effective current spread therefore depends on the neural event being measured.


Direct activation of sparse, distributed populations of cortical neurons by electrical microstimulation.

M. H. Histed and V. Bonin and R. C. Reid

Neuron  63  508-22  (2009)


http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=19709632

For over a century, electrical microstimulation has been the most direct method for causally linking brain function with behavior. Despite this long history, it is still unclear how the activity of neural populations is affected by stimulation. For example, there is still no consensus on where activated cells lie or on the extent to which neural processes such as passing axons near the electrode are also activated. Past studies of this question have proven difficult because microstimulation interferes with electrophysiological recordings, which in any case provide only coarse information about the location of activated cells. We used two-photon calcium imaging, an optical method, to circumvent these hurdles. We found that microstimulation sparsely activates neurons around the electrode, sometimes as far as millimeters away, even at low currents. Our results indicate that the pattern of activated neurons likely arises from the direct activation of axons in a volume tens of microns in diameter.